1. Field of the Invention
This invention is in the field of packages for solid-state displays. More specifically, this invention relates to a filler-in-plastic cover that provides protection and improved light-scattering characteristics.
2. Description of the Prior Art
Packages of the same general type as the one of this invention are known, and have been described in U.S. Pat. No. 3,911,430 and U.S. Pat. No. 3,786,499, both assigned to the same assignee as this invention. Such packages generally employ a supporting substrate on which are mounted a plurality of light-emitting diodes. Located over the diodes are a plurality of light pipes, one pipe for each diode, that function to transmit light from the diodes to a translucent protective cover.
Previously, in order to reduce regions of concentrated light intensity, sometimes referred to as "hot spots," some kinds of lens mechanism has been provided to disburse light from the light-emitting diodes. For example, a surface of the protective cover, such as the interior surface facing the light-emitting diodes, has been formed so that the surface comprises a plurality of concentric circles, creating what is often referred to as a fly's eye lens. As known in the art, fly's eye lenses function to disperse impinging light and reduce unwanted regions of high light concentration. FIG. 1 shows a device with a plurality of fly's eye lenses, such as lens 5.
Unfortunately, it is difficult to mass produce transparent protective covers having one surface thereof comprising fly's eye lenses. For example, in prior-art applications, use has been made of a die stamp attached to a support to form the fly's eye lenses. With high-volume use, the stamp often comes loose from its support. To prevent production problems that arise when the stamp and support separate from each other, an alternative approach, comprising use of a mold of a very hard material, such as tool steel, has been used for the die. Making a mold of tool steel is a slow and costly process because hundreds to thousands of small circles must be created in one surface of the mold. Because such a mold is slow and costly to produce, this approach lacks the flexibility desirable in semiconductor technology where many different sizes and shapes of covers are needed. Most users are reluctant to pay the high cost of numerous tool steel molds. It is also difficult using tool steel to obtain the correct fly's eye lens shape for optimum light scattering.
After a cover with fly's eye lenses is made, if a misalignment occurs between the fly's eye pattern in the cover and a segment of the alpha-numeric display, such as a numerical character, illumination of the character can be uneven. Moreover, during the time when a character is initially displayed, the fly's eye pattern sometimes can be seen due to a different light output in each point on the fly's eye surface. Furthermore, the fly's eye lens system does not eliminate enough of the "hot spot" in the center of a character, compared to the ends of a character. In addition, the fly's eye lens system tends to scatter too much of the light away from the direct viewing axis of the display, which is the most important direction in which to have high illumination.
Some prior-art approaches have made use of a combination of a fly's eye lens system and use of small light-scattering particles in the protective cover, with the small particles comprising one to four percent by weight of the protective cover. The small particles help reduce the "hot spot" problem of the fly's eye lens system but increase the scattering of light away from the direct viewing axis of the display. This combination of fly's eye lenses and light-scattering particles often reduces the intensity of the light by a factor of two. When the light-scattering particles in a protective cover are above four percent by weight, the combined effect with a fly's eye lens system results in a light intensity that is too low to be acceptable for many applications.
Therefore, a new approach is needed to scatter light through a translucent protective cover without the disadvantages of the fly's eye lens system. The approach should be easy and inexpensive to produce on a high-volume basis, it should not require a mold that is slow and costly to manufacture, it should provide great flexibility in allowing many different sizes and shapes of covers to be made, it should eliminate differences in illumination between the center and edges of a character, and it should scatter more light through the cover on the direct viewing axis than that of the fly's eye lens approach.